Question

I’m currently using this for turning an enum into a radio control,

public static MvcHtmlString RadioButtonForEnum<TModel, TProperty>(
    this HtmlHelper<TModel> htmlHelper,
    Expression<Func<TModel, TProperty>> expression)
{
    var metaData = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);

    var sb = new StringBuilder();
    var enumType = metaData.ModelType;
    foreach (var field in enumType.GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public))
    {
        var value = (int)field.GetValue(null);
        var name = Enum.GetName(enumType, value);
        var label = name;
        foreach (DisplayAttribute currAttr in field.GetCustomAttributes(typeof(DisplayAttribute), true))
        {
            label = currAttr.Name;
            break;
        }

        var id = string.Format(
            "{0}_{1}_{2}",
            htmlHelper.ViewData.TemplateInfo.HtmlFieldPrefix,
            metaData.PropertyName,
            name
        );
        var radio = htmlHelper.RadioButtonFor(expression, name, new { id = id }).ToHtmlString();
        sb.AppendFormat(
            "<label for=\"{0}\">{1}</label> {2}",
            id,
            HttpUtility.HtmlEncode(label),
            radio
        );
    }
    return MvcHtmlString.Create(sb.ToString());
}

but when trying to adapt it to enum to dropdown:

public static MvcHtmlString DropDownListForEnum<TModel, TProperty>(
   this HtmlHelper<TModel> htmlHelper,
   Expression<Func<TModel, TProperty>> expression)
{
     var metaData = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);

     var sb = new StringBuilder();
     var enumType = metaData.ModelType;
     sb.Append("<select name=\"" + metaData.PropertyName + "\" id=\"" + metaData.PropertyName + "\" > ");
     foreach (var field in enumType.GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public))
     {
          var value = (int)field.GetValue(null);
          var name = Enum.GetName(enumType, value);
          var label = name;

          foreach (DisplayAttribute currAttr in field.GetCustomAttributes(typeof(DisplayAttribute), true))
          {
                label = currAttr.Name;
                break;
          }

          var id = string.Format(
                    "{0}_{1}_{2}",
                    htmlHelper.ViewData.TemplateInfo.HtmlFieldPrefix,
                    metaData.PropertyName,
                    name
                );
          var listitem = htmlHelper.DropDownListFor(expression, name, new { id = id }).ToHtmlString();
          sb.AppendFormat(
                    "<option value=\"{0}_{1}\">{2}</option> ",
                    id,
                    listitem,
                    HttpUtility.HtmlEncode(label)
                );
     }
     sb.Append("</select>");
     return MvcHtmlString.Create(sb.ToString());
}

I get an error in the var listitem = htmlHelper.DropDownListFor line. Basically I’m not providing the correct information in the method. Could anyone shed any light on this issue?

Answer 1

You can use a static helper that turns enums into select lists.

I’ve blogged about it here:

http://jnye.co/Posts/4/creating-a-dropdown-list-from-an-enum-in-mvc-and-c%23

The helper (this will get the value of the description attribute if present):

public static class EnumHelper
{
    public static SelectList SelectListFor<T>(T? selected)
        where T : struct
    {
        return selected == null ? SelectListFor<T>()
                                : SelectListFor(selected.Value);
    }

    public static SelectList SelectListFor<T>() where T : struct
    {
        Type t = typeof (T);
        if (t.IsEnum)
        {
            var values = Enum.GetValues(typeof(T)).Cast<Enum>()
                             .Select(e => new { Id = Convert.ToInt32(e), Name = e.GetDescription() });

            return new SelectList(values, "Id", "Name");
        }
        return null;
    }

    public static SelectList SelectListFor<T>(T selected) where T : struct 
    {
        Type t = typeof(T);
        if (t.IsEnum)
        {
            var values = Enum.GetValues(t).Cast<Enum>()
                             .Select(e => new { Id = Convert.ToInt32(e), Name = e.GetDescription() });

            return new SelectList(values, "Id", "Name", Convert.ToInt32(selected));
        }
        return null;
    }

    public static string GetDescription<TEnum>(this TEnum value)
    {
        FieldInfo fi = value.GetType().GetField(value.ToString());

        if (fi != null)
        {
            DescriptionAttribute[] attributes =
                (DescriptionAttribute[])fi.GetCustomAttributes(
                    typeof(DescriptionAttribute),
                    false);

            if (attributes.Length > 0)
                return attributes[0].Description;
        }

        return value.ToString();
    }
}

This helper will enable you to convert an enum to a select list in just two lines.

In your controller:

ViewBag.TypeDropDown = EnumHelper.SelectListFor(field);

In your view:

@Html.DropDownList("TypeDropDown")